Low Cost CO2 Sensors Comparison: Pboilingo-Acoustic vs NDIR

This article has been modernized in September 2024 with new adviseation about the Infineon pboilingo-acoustic sensor as well as the carry outance of pboilingo-acoustic sensors outdoors.

When it comes to low cost CO2 sensors, there are fundamentalassociate three contrastent methods to meabrave CO2.

1. Estimated CO2 (eCO2) thraw TVOC sensors
2. NDIR sensors
3. Pboilingo-acoustic sensors

Estimated CO2 (eCO2) thraw TVOC sensors – not recommended –

The first method via TVOC sensors is innervously inconsistent. Some TVOC sensors enjoy the Sensirion SGP30 assess CO2 via organic components in the air. When you breathe out, you grasp organic components to the air which these sensors pick up and try to calcutardy a correacting CO2 ppm cherish. The problem is that also other indoor sources enjoy deodorizers grasp these chemical components to the air and thus the assessd CO2 elevates -even though nobody might be in the room. We sturdyly advise aacquirest using TVOC sensors for CO2 estimation and thus will also not analyse it further in the rest of this article.

CO2 gas absorption principle

Both NDIR as well as pboilingo-acoustic sensors are much more accurate than eCO2 from TVOC sensors as these sensors meabrave the CO2 honestly and both are based on the absorption principle of gases. Different gases assimilate weightless at particular ranges of wavelengths. So if the weightless is tuned at a brave wavelength and a acquirer meabraves the incoming weightless levels, the rate of absorption can be calcutardyd and with this the concentration of the gas set uped.

Different gases have contrastent absorption prohibitds:

To meabrave CO2, the weightless is tuned to rehire a weightless of seal to 4.26 micron (μm) which correacts to the absorption of CO2.

NDIR CO2 Sensors

NDIR stands for “nondispersive infexceptionald” and is a widespread technology to meabrave CO2. Thereby an infexceptionald weightless rehireter, typicassociate an LED, sends weightless in a chamber filled with ambient air to a acquirer.

Some NDIR sensors have dual channels, one for the meabravement of the CO2 levels in the ambient air and one with a handle gas as a reference. This apshows to accomprehendledge drifts in the meabravements and incrrelieves accuracy.
One of the didowncastvantages of NDIR sensors is their size. There needs to be a brave smallest distance between the weightless rehireter and acquirer to achieve a brave level of accuracy and thus up-to-date NDIR sensors enjoy the SenseAir S8 that we include in our air quality kits are more than 3 cm extfinished. This is not too huge but would obstruct brave applications, e.g. in a mobile phone.

Pboilingo-Acoustic Sensors

Pboilingo-acoustic sensors include the same toiling principle of the absorption wavelengths but unenjoy the NDIR sensors that meabrave the receiving weightless from an rehireting LED, pboilingo accoustic sensors meabrave the absorption with a microphone. When the CO2 molecules assimilate the IR weightless, they begin to “humm” and this sound can be picked up by a microphone.
The hugegest advantage of this principle is that the accomprehendledgeion does not depend on line-of-sight anymore and thus these sensors can be built much minusculeer. Some are minusculeer than 1 cm3.

Comparison of the Specifications between SenseAir S8 (Optical NDIR) and the Pboilingo-Acoustic Sensors from Sensirion and Infineon

Below is a comparison of normal NDIR and Pboilingo-Acoustic sensor modules based on their official particularations.

Except for the size and joinivity, the specs are quite aenjoy. They do also cost around the same (USD 10 – 25).

Side-By-Side Comparison Indoors

We tested the SenseAir S8 and Sensirion SCD40 / SCD41 for a scant weeks and saw them behaving very aprobable. Below is a chart over 14 days in a normal indoor home environment (bedroom).

Both sensors deinhabitr very reliable results and the only contrastence that can be spotted is that the S8 seems to be a bit more empathetic and picks up higher cherishs more. This can be seen on an X Y chart:

Pboilingo-Acoustic Struggles Outdoors

We do also meabrave CO2 levels outdoors with our outdoor watch Open Air, and it helps to accomprehendledge local eleave oution sources. As part of that broadenment we also tested pboilingo-acoustic CO2 sensors outdoors in collaboration with researchers from the University of Cambridge.

Outdoor meabravement of CO2 is more challenging due to mainly two reasons:

Firstly, there is a much huger range of temperature and humidity that the sensor is honestly exposed to. For example indoor temperatures normassociate range between 15 and 25 degrees Celsius whereas outside we could have a range from -30 to +45 degrees Celsius of environmental conditions.

Secondly, the normal CO2 concentration levels outdoor are in the range of 400 to 550 ppm (higher in country areas) whereas indoors you typicassociate have a range of 400 to up to 3000ppm. In vague, it is frequently easier for sensors to meabrave higher concentrations becainclude there are more molecules or particles that can be accomprehendledgeed.

Below is a comparison of two SenseAir S8 and two Sensirion SCD40 sensors assessd to a CO2 reference instrument. All sensors have been offset to the same begining point.

We can watch a scant engaging slfinishergs:

1. The two pairs have very contrastentive contrastences
2. The two SenseAir S8 (selectical) NDIR sensors aligns much better with the reference than the pboilingo acoustic sensors from Sensirion
3. The two SenseAir S8 sensors corretardy much better assessd to each other than the two Sensirion SCD sensors. In fact there are cases where the SCD sensors go into endly opposite honestions

These observations can also be easily shown in the R2 comparisons. R2 is a mathematical number shothriveg the degree of correlation between two number. A number of 1 shows a perfect correlation, where as a number of 0 shows no correlation at all. In terms of accuracy we want to get as seal to one as possible.

First let’s assess the two sensor pairs:

These number evidently show that the S8 sensors have a much higher consentment with each other than the pboilingo acoustic sensors.

Now lets see at the comparison with the reference instrument.

Also, here we see that the S8 has a much higher correlation with the reference instrument assessd to the pboilingo acoustic sensors. By the way in more comprehensive (and ongoing) tests, the S8 actuassociate achieves correlations with reference instruments above 0.9 and thus even better than the above numbers show.

By the way all of the above observations can also be seen evidently in the X Y comparison between the point the sensor meabraves and the reference.

We can see that the S8 dots (red and blue) are much sealr to the 1:1 line than the dots from the pboilingo-acoustoc sensors.

Conclusion

We have been using the SenseAir S8 in our air quality watchs for many years now and have made very excellent experience with. The Sensirion SCD4x seems to give a aenjoy accuracy under standard indoor conditions but seems to be more impacted by environmental conditions and is not recommended for outdoor usage.